Web Animations CSS: remove unnecessary class CSSValue declaration

The code already #includes "core/css/CSSValue.h" so no need to declare
the class a second time.

Review URL: https://chromiumcodereview.appspot.com/24072007

git-svn-id: svn://svn.chromium.org/blink/trunk@157995 bbb929c8-8fbe-4397-9dbb-9b2b20218538